Since the amount of wastewater has been quickly increased in recent years, environmental contamination caused by the wastewater became a serious problem. Various efforts to efficiently treat industrial wastewater have been developed.
Generally, physical/chemical wastewater treatment techniques have been typically used for treating industrial wastewater of high sludge content having heavy-metal contaminants. In the physical/chemical wastewater treatment techniques, sludge with heavy-metal contaminants is separated from water using a variety of chemicals or polymeric coagulants and, then, the sludge is dewatered using various dewatering devices, to thereby make sludge cakes which are then dried to be buried under the ground to be burnt. In the meantime, the supernatant separated from the sludge is treated through biological treatment techniques or physical/chemical treatment techniques.
In sewage treatment plants that are representative examples of facilities operated through biological treatment technologies, the BOD concentrations of the sewage introduced into the sewage treatment plants have been increased according to an improvement in the living standard. Thus, the content of microorganisms in the sludge of the sewage is increased to disturb the precipitation of the sludge. Furthermore, in recent years, existing sewage pipelines in which rainwater and sewage flow together within a pipeline without being separated have been change with new sewage pipelines in which rainwater and sewage separately flow through separate pipelines. Due to such a change of the sewage pipelines, the content of inorganic solids in the sludge of the sewage introduced into the sewage treatment plants is reduced, so that the sludge precipitation effect is reduced to cause a reduction in the density of the sludge. Thus, the sewage treatment plants suffer from a reduction in the operational performances of their dewatering devices and sludge digesters (facilities to reduce the quantity of sludge).
FIG. 12 schematically shows the construction of a conventional system for sludge treatment.
As shown in FIG. 12, water which has been separated from precipitated sludge of sewage due to a difference of specific weight between solids and liquids in a sedimentation basin 11 overflows from the basin to be drained, while the precipitated sludge is transferred to a sludge storage tank 12 through a first transfer line 31 by a pumping operation of a first pump 21. In the above state, the density of the precipitated sludge which is transferred to the sludge storage tank 12 varies according to variations in the temperatures and sludge contents, a change of season, a number of sludge transferring cycles, etc.
The sludge is, thereafter, transferred from the sludge storage tank 12 to a mixing flocculation tank 13 through a second transfer line 32 by a pumping operation of a second pump 22. In the above state, a coagulant used for separating water from the sludge is dissolved in water in a coagulant dissolution tank 14 by a first agitator 41, and, thereafter, fed to the mixing flocculation tank 13 through a third transfer line 33 by a pumping operation of a third pump 23. In the mixing flocculation tank 13, the sludge and the coagulant are mixed together by a second agitator 42, thus producing flocs. The flocs are, thereafter, transferred to a dewatering device 15 through a fourth transfer line 34, thus being dewatered in the dewatering device 15. To secure a stable operation and automation of the dewatering device 15, the flocculated sludge is required to maintain a predetermined constant density and a predetermined moisture content thereof, and to be in a state in that solids are sufficiently separated from liquid before the sludge is transferred to the dewatering device 15.
However, the amount of the coagulant which is added to the sludge transferred to the mixing flocculation tank 13 cannot be appropriately controlled due to a variety of variables, such as a variation in the sludge sizes caused by the variations in the sludge contents and temperatures, the change of season, and the activated states of microorganisms, sludge storage time and sludge storage state of the sludge storage tank, particularly in the case of a biological precipitation of the sludge. Furthermore, the density of the sludge transferred to the sludge storage tank 12 varies according to the number of the sludge transferring cycles to transfer the precipitated sludge from the sedimentation basin 11 to the sludge storage tank 12, and a seasonal variation in the sludge precipitation state of the sedimentation basin 11, etc. Thus, the variation in the density of the flocculent sludge transferred from the mixing flocculation tank to the dewatering device 15 may excessively vary to cause a reduction in the operational performance of the dewatering device 15, thus disturbing a proper operation of the dewatering device 15.
FIG. 13 schematically shows the construction of a conventional sludge concentration system according to another embodiment of the related art.
As shown in FIG. 13, the sludge precipitated in a sedimentation basin 51 is transferred to a centrifugal concentration device 52 through a first transfer line 71 by a pumping operation of a first pump 61. The centrifugal concentration device 52 concentrates the sludge to a predetermined level. Thereafter, the concentrated sludge is transferred to a sludge storage tank 53 through a second transfer line 72 by a pumping operation of a second pump 62.
Thereafter, the sludge is transferred from the sludge storage tank 53 to a mixing flocculation tank 55 through a third transfer line 73 by a pumping operation of a third pump 63. In the above state, a densitometer 57 is installed on an intermediate portion of the third transfer line 73 to measure the density of the transferred sludge.
Furthermore, a part of a coagulant, dissolved in water in a coagulant dissolution tank 54 by a first agitator 81, is fed to the mixing flocculation tank 55 through a fourth transfer line 74 by a pumping operation of a fourth pump 64. In the above state, the amount of the coagulant to be fed is controlled through a feedback control method in which the amount of the coagulant to be added is determined based on the density of the sludge measured by the densitometer 57. In the mixing flocculation tank 55, the sludge and the coagulant are mixed together by a second agitator 82, thus producing flocs. Thereafter, the flocs are transferred to a dewatering device 56 through a fifth transfer line 76, so that the dewatering device 56 dewaters the flocs.
When the sludge, which has been transferred from the sedimentation basin 51 through the first transfer line 71, is concentrated in the centrifugal concentration device 52, the sludge concentration process is executed using a centrifugal force determined based on both the density of the sludge and the precipitation characteristics of the sludge precipitated in the sedimentation basin 51. Thus, the density of the sludge may excessively vary and, furthermore, the sludge may not be concentrated to a desired level. Furthermore, the amount of the coagulant fed to the mixing flocculation tank 55 may vary according to the above-mentioned various parameters as well as the density of the sludge measured by the densitometer 57. Therefore, the densitometer 57 is not efficiently used, so that the conventional sludge concentration system is not practically used.
As described above, it is very difficult to automatically feed an appropriate amount of chemicals to the mixing flocculation tank due to the above-mentioned several causes. Furthermore, due to the variation in the density of the sludge transferred from the mixing flocculation tank to the dewatering device, the operation of the dewatering device cannot be appropriately managed. Thus, the system is operated depending on the sense of an operator of the system. This results in a various problems, such as a consumption of an excessive amount of chemicals, an inappropriate operation of the dewatering device, and an ineffective treatment of the sludge.